High%Strain+Rate% Characterizaonof Magnesium%Alloys · 2012-12-27 · High%Strain+Rate%...
Transcript of High%Strain+Rate% Characterizaonof Magnesium%Alloys · 2012-12-27 · High%Strain+Rate%...
High Strain-‐Rate Characteriza1on of Magnesium Alloys
Srdjan Simunovic Oak Ridge Na4onal Laboratory October 5, 2011
Project ID # LM032 This presenta4on does not contain any proprietary, confiden4al, or otherwise
rstricted informa4on
2 Managed by UT-Battelle for the U.S. Department of Energy
Objec1ve
• Provide enabling technologies for use of lightweight materials (Mg alloys) in automobiles by the development of: – High strain rate test methods – Methods for characteriza4on of material property degrada4on (damage) evolu4on under high rates
– Methods for failure characteriza4on at high rates – Databases of material proper4es – Cons4tu4ve models for FEM simula4ons
• Technology developed in this project is directly applicable to Mg alloys and other lightweight materials.
Relevance
3 Managed by UT-Battelle for the U.S. Department of Energy
Relevance to VT Program • The current designs with lightweight materials result in overdesigned components in order to compensate for the uncertain4es in the deforma4on and failure mechanisms.
• Reducing uncertain4es in component design greatly improves the overall vehicle system reliability and enables weight reduc4on (Technical Target: Vehicle Weight).
• Vehicle designers need (Barriers C and G): – Mechanical proper4es under impact, damage and failure
characteriza4on, material and failure models, FEM technology – Methods for material characteriza4on under impact – Very limited material data and models are currently available for
crashworthiness of Mg alloys
• This project develops enabling technologies and material data for use of Mg alloys in automo1ve structural applica1ons.
Relevance
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Crashworthiness Tests
• Impact energy dissipa4on in Mg tube occurs by different process than in the conven4onal automo4ve materials.
• Vehicle designers need to understand ini1a1on and evolu1on of internal state and failure processes in Mg alloys as func1ons of loading type and loading rates.
HSLA Mg Alloy
Technical Approach
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Strain Rates in Automo1ve Design • Low (quasi-‐sta1c) rate tests -‐ en4re system is in equilibrium at all 4mes • Fast rate tests -‐ single impact pulse travels through the system • Intermediate rate tests (between 1/s and 1000/s) -‐ mul4ple wave reflec4ons in the system – Difficult to establish dynamic equilibrium in the sample and the system – These rates are important because maximum strain rates in automo1ve crash are in
the interval of 10-‐1000/s (also important for forming) • It is essen4al to: – Reduce mass in the system – Develop lightweight load cells and sensors – Understand and control oscilla4ons in the system – Combine mul4ple measurement techniques for the same data
• Intermediate strain rate tests have not been standardized – Ac4ve research field
• This project develops new tes1ng methodologies and material informa1on for the strain rates of interest in vehicle design.
Technical Approach
6 Managed by UT-Battelle for the U.S. Department of Energy
Approach/Strategy • Develop new methods and procedures for measurement of material behavior at intermediate strain rates. – Develop test methods, loading equipment, control, sensors, specimens,
fixtures.
• Test measurements must be based on mul4ple sensor types. – We must have mul4ple data sources for each measured quan4ty.
• Verify measurements throughout the range of strain rates of interest. – As some sensor types reduce accuracy with increasing speeds, new sensors
must gradually take over. – Validity of a new sensor type must be established in the transi4on region.
• Develop new methods for strain-‐interrupted tests at high rates. – Essen4al for inves4ga4on of strain-‐rate dependency of damage evolu4on. – It is easier to speed up than to instantly stop, especially at 500 in/sec.
• Inves4gate forma4on and growth of voids in Mg alloys using microscopy for strains and strain rates of interest.
• Distribute data as soon as possible. Technical Approach
7 Managed by UT-Battelle for the U.S. Department of Energy
Custom Designed Hydraulic System Max Velocity=700 in/s (18.5 m/sec) over approx. 4 in (100 mm) Range Load Capacity : 9000 lbs (40 kN) sta4c, 5500 lbs (25 kN) dynamic Total Stroke: 15.5 in (400mm) Working Stroke: approx. 7.0 in (175 mm) with slack adapter in the load train Control: MTS 407 servo -‐ hydraulic controllers, with external command signal (drive file). In-‐house developed synchroniza4on and DAQ systems.
Materials and Test Instrumenta1on
High speed digital imaging system Max. Frame rate: 1000000 fps 3D imaging capability for full field displacement map
Materials Tested: AZ31 sheet metal AM60B cast (top hat) AM60B unprocessed Materials for test development (AHSS)
Control and synchroniza1on of mul1ple data sources are essen1al for accuracy at high speeds.
Technical Approach
8 Managed by UT-Battelle for the U.S. Department of Energy
Tests for Intermediate Strain Rate Regime Use Mul1ple Sensors
Specimen and sensor schema1cs
• Standard load cells loose accuracy at high rates due to iner4a of the devices.
• Strain gages on the specimen are used for strain and stress (tab region) measurements.
• Op4cal strain measurements from high speed camera are used for strain measurements across the specimen.
• Measurement from different sensors is compared for different strain rates in order to establish correla4ons and es4mate errors.
Technical Approach
Test Instrumenta1on and Setup
9 Managed by UT-Battelle for the U.S. Department of Energy
0
0.1
0.2
0.3
0.4
0.1 0.2 0.3 0.4 0.5
strain
time, s
stroke calculated strainVIC measrued strain for gage sectionvic measured strain inside necking region
New Test Setup Provides for Accurate Measurements of High Strains and Rates
• Conven4onal methods of calcula4ng strains from stroke are not accurate. – At low strain rate (1 /s), strain calculated from stroke tends to overes4mate
the average strain of the gage sec4on. – At high strain rate (500 /s), strain calculated from stroke tends to
underes4mate the average gage sec4on strain.
• Digital Image Correla1on (VIC) enables measurement of strains well beyond the range of fast-‐response bondable foil gages.
1 /s
0
0.1
0.2
0.3
0.4
0.5
0.0244 0.0245 0.0246 0.0247 0.0248
strain
time, s
stroke calculated strainVIC measrue strain in gage sectionVIC measured strain in necking regionSG in gage section
500 /s
Technical Accomplishments
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New Strain Rate Tes1ng Method
• Conven4onal approaches give inaccurate measurements that need to be filtered and cannot provide data for small strains and for high strain rates.
• We accomplished significant improvement in measurements of stresses, strains and strain rates in the intermediate strain rate regime.
Conven4onal method (LVDT/Load washer)
AZ31 at 100/s
New method New method
LVDT/Load washer
AZ31 at 500/s
Technical Accomplishments
11 Managed by UT-Battelle for the U.S. Department of Energy
0
100
200
300
400
0 0.05 0.1 0.15 0.2 0.25
Stress, M
Pa
Strain
1248 /s860 /s547 /s71 /s10.5 /s1 /s
Strain Rate Sensi1vity of Mg Alloys in Tension
• Raw data, no filters have been used • Tests are run on the same equipment • Data can be processed and downloaded from the project’s web site
0
100
200
300
400
0 0.05 0.1 0.15 0.2 0.25
Stress, M
Pa
Strain
862 /s743 /s57 /s8 /s0.7 /s
AZ31 AM60B
12 Managed by UT-Battelle for the U.S. Department of Energy
0
50
100
150
200
250
0 0.02 0.04 0.06 0.08 0.1
Shear stress, M
Pa
shear strain
0.74/s 0.8/s 0.9/s
7.5/s 7.6/s 126.8/s
146/s
0
50
100
150
200
250
0 0.02 0.04 0.06 0.08 0.1
shear stress, M
Pa
shear strain
1/s0.85/s8.8/s7.9/s167.5/s111/s
Strain Rate Sensi1vity of Mg Alloys in Shear
• Raw data, no filters have been used • The tests show reasonable strain distribu4ons in the test specimens. AM60B fails in fracture mode. • Test data correlates well with the tension tests.
AZ31
AM60B
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We have Designed New Test Fixtures and Specimens for Prescribed Strain Applica1on at High Strain Rates
• Difficult to control • Problems increase with speed • Requires a large number of tests • Scrapped, went with new approach
• Reduces the number of tests and specimens • Shape design is determined by modeling • No added mass, simpler, cheaper method • Inven4on applica4on in process
Mul1-‐gage Tapered shape Type I Type II
A^empts at New Fixture Designs New Specimen Designs
Technical Accomplishments
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The New Test Generates Mul1ple Strain Rates in a Single Specimen
• The new method generates smoothly increasing strain path to the final strain under reasonably constant strain rate. • Further test development is planned for in order to improve the control of strain and strain rate distribu4on.
Technical Accomplishments
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Damage Analysis in Mg Alloys
• The void sta4s4cs are generated by image processing. – Data can be used for calibra4on of micromechanics-‐based material and
failure models for FEM simula4ons.
Original Image Discovered Voids Void Size Distribu1on
Technical Accomplishments
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Void Profile Examples for AM60 • Results are generated by the interrupted strain method • The tests show that void nuclea4on and growth (damage) intensifies with strain rates for AM60. – More test are needed
for different ini4al porosi4es
Technical Accomplishments
ε = 100/s .
ε = 500/s .
0% 5% 10%
100x ε = 1/s .
Strain
Strain ra
te
ε = 100/s .
17 Managed by UT-Battelle for the U.S. Department of Energy
Collabora1on and Coordina1on with Other Ins1tu1ons • Collabora4on with Mg Front End R&D Project – Materials supplied by OEMs
• Technology Transfer – Test data distributed via www
• hmp://thyme.ornl.gov/Mg_new
– Users can select/view/process data and download it for FEM material models
– Direct communica4on to OEMs
• Educa4on – Two postdoctoral students – 8 student interns so far – 9 summer students for 2011
18 Managed by UT-Battelle for the U.S. Department of Energy
• In-‐depth characteriza4on of damage evolu+on in Mg alloys of interest to automo4ve designers at various strain rate condi4ons, loadings and ini4al microstructures. – Develop mechanics-‐based methods for damage characteriza4on.
• Advance technology for intermediate strain rate tes4ng in tension, shear and mul+-‐axial loading configura4ons. – Essen4al for development and calibra4on of advanced material models for lightweight
materials. The damage models require wide region of stress states and loading paths. – Support development of forming technologies and new material processes such as
asymmetric rolling for Mg alloys
• Development of methods for characteriza4on of fracture/failure proper4es of Mg sheet form.
Proposed Future Work
Stress Triaxiality
19 Managed by UT-Battelle for the U.S. Department of Energy
Summary • We have developed and demonstrated test methods for characteriza4on of materials under strain rates typical of vehicle crash. – Tension and shear configura4ons were developed. – Tests provide much needed data for Mg alloy applica4ons. – Tests are used for material model development and calibra4on.
• A new method for interrupted strain rate tests have been developed that enables characteriza4on of material internal state under intermediate strain rates. • The developed methods can be used to address complex problems of characteriza4on of fracture and failure in sheet materials. • Technology transfer occurs via www
http://thyme.ornl.gov/Mg_new!